EOS EOS,

Eos, Vol. 86, No. 22, 31 May 2005

EOS

E O S , T R A N S A C T I O N S , A M E R I C A N G E O P H Y S I C A L U N I O N

Hydrothermal and Volcanic Activity Found on the

Southern Mid-Atlantic Ridge

PAGES 2 0 9 , 2 1 2

The process of plate accretion at mid-ocean ridges, o n c e thought to o c c u r in a relatively simple, magmatic system, has b e e n shown in recent years to possess unexpected layers of complexity [e.g.,Cannat, \9%;Escartin and Lin, 1998; Jokat et al, 2003; Michael et al, 2 0 0 3 ] . Particularly at lower spreading rates, the magma supply to s o m e or all of the ridge decreases, with the plate spreading motion being taken up instead on faults.

The b a l a n c e b e t w e e n these magmatic and tectonic processes governs such features as the topography, seismic activity location of hy­

drothermal vents, and degree of c h e m i c a l ex­

c h a n g e between crust and o c e a n at spreading axes. It therefore has important implications for the hydrothermal marine biosphere and global c h e m i c a l budgets.

With increasing tectonism at lower spread­

ing rates c o m e s increasing heterogeneity in the accretion process. This m e a n s that ridge crest surveys n e e d to b e correspondingly lengthened in order to make meaningful and statistically significant statements about the role of these ridges in global lithospheric and o c e a n o g r a p h i c processes. Unfortunately it is exactly the slow spreading ridges which, up to present, have b e e n the least studied globally, with only - 1 0 % of their total length having b e e n surveyed to date for hydrothermal activ­

ity, for example [Baker and German, 2 0 0 4 ] .

Six-Year Effort on the Mid-Atlantic Ridge

To address this need, the German S c i e n c e Foundation (DFG) initiated in O c t o b e r 2003 a six-year multi-cruise study of a 450-km-long stretch of slow spreading axis in the South Atlantic b e t w e e n 7° and 11°S near the Island of Ascension. Previous studies [Bruguier et al, 2003;Minshull et al., 1998] had divided this area into four major segments, A1-A4 ( s e e

B Y C . W . DEVEY, K . S. LACKSCHEWITZ, AND E. BAKER

Figure 1). The widely varying topography and crustal thicknesses suggested that rates of m a g m a supply differ greatly from o n e segment to another.

S o m e of the aims of the DFG study are to establish the characteristic spacing between hydrothermal vents in the area and the mag- matic/tectonic features which control their location; to determine the total hydrothermal heat, water, and c h e m i c a l fluxes from the vents; and to e x a m i n e how the fauna associ­

ated with the vents relates to those known from other vent areas around the globe.

The first cruise in the series (with the Ger­

man research vessel Meteor, cruise n u m b e r M 6 2 / 5 ) returned to port at the end of D e c e m ­ ber 2004 with new insights into the volcanol- ogy and t e c t o n i c s of the southern Mid-Atlantic Ridge ( M A R ) , a n d the first detailed mapping of a hydrothermal plume south of the equator in the Atlantic.

Varying Volcanology and Tectonics Along Axis

During a 12-day deployment of the Brit­

ish deep-towed side-scan sonar system TOBI (Towed O c e a n Bottom Instrument),6000 k m² of the spreading zone between the Ascension and B o d e Verde Fracture Zones (Figure 1) were imaged for the first time, yielding evi­

d e n c e for both magmatically and t e c h n i c a l l y dominated spreading.

Segment A l appears to have a low m a g m a supply—the axial valley is deep,volcanism is c o n c e n t r a t e d on small within-axis volca­

nic ridges, and these ridges are s o m e t i m e s topped by discrete small v o l c a n o e s with well developed calderas. Optical backscatter in­

formation from miniature a u t o n o m o u s plume recorders (MAPRs) attached to the TOBI ve­

hicle and its tow wire showed several signs of turbid hydrothermal plumes within the water column, in s o m e c a s e s confirming previous indications from a conductivity-temperature- depth (CTD) profile in the area [German et al, 2 0 0 2 ] .

Segment A2 showed s o m e of the youngest- looking flows (Figure l b and c ) . Both off-axis (Figure l b ) and on-axis (Figure l c ) flows are

VOLUME 86 NUMBER 22 31 MAY 2005

PAGES 209-216

s e e n to blanket axial fault scarps. T h e on-axis flow (Figure l c ) does not appear to have b e e n fractured by any fault planes large enough to b e visible at the resolution of the TOBI images (6 m m e a n pixel resolution).

Segment A3 is the shallowest segment, with crustal thicknesses up to 11 km [Bruguier et al,2003]. T h e axial region is characterized by extensive sheet flows and the virtual a b s e n c e of "hummocky terrain," which is the surface expression of pillow v o l c a n o e s and a typical feature of slow spreading ridges.

The jump to segment A4 is a c c o m p a n i e d by a large increase in axial depth and a return to volcanic features similar to those s e e n on seg­

ment A l . Despite the fact that A4 is labeled as o n e segment, the side-scan data show several individual v o l c a n i c segments, separated from o n e another by nontransform displacements of up to 8 km, within the axial valley

Evidence for Multiple Hydrothermal Sites Although there is e v i d e n c e for as many as 24 venting sites between 15°N and 38°N on the northern MAR [Baker and German, 2 0 0 4 ] , information on the locations of active hydro- thermalism on the southern MAR is virtually nil, with the exception of s o m e total dissolved m a n g a n e s e (TDMn) a n o m a l i e s in the water c o l u m n found by German et al [ 2 0 0 2 ] .

During the Meteor cruise, strong e v i d e n c e was found for hydrothermal activity on a topo­

graphic high that rises to 2900 m depth from the rift valley floor at 3500 m depth between 8°17'S and 8°19'S to the east of the tip of seg­

ment A2 ( s e e star in Figure 1 ) . High m e t h a n e concentrations (up to 115 n a n o m o l e s per litre, or nmol/L) together with layers of increased light scattering peaking at 2700 m depth in the vicinity of 8° 18'S, 13°31'W (Figure 2 ) , indicate the p r e s e n c e of venting in this area.

Although b l a c k smokers have not yet b e e n directly observed, m a n g a n e s e concentrations of up to 25 nmol/L, c o i n c i d e n t with the peak in m e t h a n e concentration and light scat­

tering (Figure 3 ) , c o n f i r m the hydrothermal nature of this water anomaly Additionally a temperature anomaly of 0.14°C 2 m above the seafloor, found during a remotely operated vehicle (ROV) dive at the western flank of the area, is probably related to heat transfer from a hydrothermal system b e l o w the surface. This higher temperature has c a u s e d pervasive local alteration of rocks and sediments. This region has b e e n n a m e d the"Nibelungen Field," after an ancient German myth of subterranean dwarves (Nibelungen) hoarding riches.

Fig. 1. Bathymetry and selected side-scan images of the axial region near Ascension Island. Red lines on side-scan images and grey lines on the bathymetric map mark the approximate trace of the plate boundary. Lighter shades on the side-scan images denote areas of high sonar reflectivity (hard, unsedimented seafloor or fault scarp), (a) Isolated volcanic cones in segment A1 seen on a postprocessed image made by combining two towed ocean bottom instru­

ment (TOBI) passes. The image width is -10 km. (b) Young, off-axis lava flows on the eastern flank of segment A2, which cover some prominent faults and appear to pond against the large fault scarp on the right edge of the picture. The unprocessed image is direct from the TOBI recorder. The image width is -6 km. (c) Postprocessed image of a large (-10 km2) lava flow in the axial valley of segment A2. Note the apparent chain of volcanic mounds just left of the plate boundary trace in the middle of the picture. The image width is -10 km. Original color image appears at the back of this volume.

Eos,Vol. 86, No. 22, 31 May 2005

Distance (km)

Fig. 2. Methane data from the "Nibelungen Field"area, along a south-north transect from 8°20' to 8°15'S at ~13°30'W Black dots indicate water samples used to construct the Figure. Methane values are presented as nmol/L; white lines denote surfaces of constant density (isopycnals) in sigma units (kg/m³ in excess of the density of pure water). Original color image appears at the back of this volume.

Two other hydrothermal plumes were also detected. Along an east-west CTD transect at 8°10'S (north of the Nibelungen Field) a clearly defined plume in methane concentra­

tion (up to 9.7 nmol/L) was identified at 2000 m water depth at 13°28'W, suggesting a hydro- thermal source probably located at the west­

ern slope of the rift valley Farther north, the TOBI/MAPR survey identified a 100-m-thick plume layer centered at 2600 m depth, extend­

ing from 7 ° 5 8 ' t o 8°02'S near 13°26.5'W While the depth range of this plume overlaps the

"Nibelungen" plumes, its location in a differ­

ent segment suggests it arises from a separate source.

The initial surveys on this ~450-km-long ridge section identify a minimum of three vent sites, or 0.7 sites per 100 km, which is consis­

tent with previous estimates of site frequency along the northern MAR. Excessive along-axis relief (Figure 1), c o m b i n e d with a typical TOBI towing altitude of 4 0 0 - 8 0 0 m above bottom, could have hidden s o m e plumes from the survey which would make this estimate con­

servative.

This study is o n e of the most detailed yet undertaken along the southern MAR, and illustrates the efficiency of melding geophysi­

cal and water column surveys into a single operation. Applying this technique to other segments would allow more robust estimates of the importance of Atlantic hydrothermal circulation at a global scale to b e made, and the importance of these sites as biological step­

ping-stones that link the North Atlantic to the other major o c e a n basins to b e determined.

Light scattering, A N T U [V]

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

• Downcast o Upcast

> ^{* V}

Concentration [nM]

40 60 80 100

Fig. 3. Light scattering sensor, methane, and total dissolved manganese (TDMnJ data from water sampling station 1230 at 8°18.00'S and

13°3L00'W.

Acknowledgments

We thank Captain N. J a k o b i and crew for the excellent support during Meteor cruise M62/

5. The cruise and postcruise scientific work was supported by the German S c i e n c e Foun­

dation (DFG). The scientific party includes B.

Bannert, 0 . Bislich, G. Engemann, L. Fowler, PGunnewig, D. Huttig,Y-G. Kim, A. Klugel, A. Knee, A. Koschinsky A. Ksienzyk, B. Kuhrig,T. Kuhn, D Matthew, C. Mertens, T. Mosch, B. Murton, N. Nowald, H. Paulick, V Ratmeyer, M. Reuter, I. Rousse, 0 . Schmale, W Schmidt, FSchubotz, M. Schroder, C. Seiter, K.

Stange, J. Stecher, U. Stober, S. Storm,

J.Sultenfuss,T.Teichgraber,PO.Thierer,S.Tille,S.

Tyler,WWalter,PWefers,and F Zielinski.

References

Baker, E.T., and C. R. German (2004),On the global distribution of hydrothermal vent fields, in Mid- Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans, Geophys. Monogr. Ser., vol. 148, edited by C. R. German et al.,pp. 245-266, AGU,Washington, D. C.

Bruguier, N. I.,T. A. Minshull, and J. M. Brozena (2003), Morphology and tectonics of the Mid- Atlantic Ridge, 7°-12°S, J Geophy. Res., 108(B2), 2093, doi:10.1029/2001JB001172.

Cannat,M. (1996), How thick is the magmatic crust at slow spreading oceanic ridges?,J. Geophys. Res., 101 (B2), 2847-2857.

Escartin,J.,and J. Lin (1998),Tectonic modification of axial crustal structure: Evidence from spectral analyses of residual gravity and bathymetry of the Mid-Atlantic Ridge flanks,Earth Planet. Sci. Lett., 754,279-293.

German, C. R.,D. PConellyA.J. Evans, and L. M. Par­

son (2002), Hydrothermal activity on the southern Mid-Atlantic Ridge,Eos Trans. AGU, 55(47), Fall Meet. Suppl.,Abstract V61B-1361.

Jokat,W,0. Ritzmann.M. C. Schmidt-Aursch, S. Drachev,S. Gauger.and J. Snow, (2003), Geophys­

ical evidence for reduced melt production on the Arctic ultraslow Gakkel mid-ocean ridge, Nature, 423,962-965.

Michael, P J., et al. (2003), Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge,Arctic Ocean, Nature, 423,956-961.

Minshull.T A.,N. J. Bruguier, and J. M. Brozena (1998), Ridge-plume interactions or mantle heterogeneity near Ascension Island, Geology, 26(2), 115-118.

Author Information

C. W Devey and K. S. Lackschewitz, IFM-GEOMAR, Leibniz Institute for Marine Sciences, Kiel, Germany;

and E. Baker, Pacific Marine Environmental Labora­

tory, Seattle, Wash.

Page 209

Fig. 1. Bathymetry and selected side-scan images of the axial region near Ascension Island. Red lines on side-scan images and grey lines on the bathymetric map mark the approximate trace of the plate boundary. Lighter shades on the side-scan images denote areas of high sonar reflectivity (hard, unsedimented seafloor or fault scarp), (a) Isolated volcanic cones in segment A1 seen on a postprocessed image made by combining two towed ocean bottom instru­

ment (TOBI) passes. The image width is -10 km. (b) Young, off-axis lava flows on the eastern flank of segment A2, which cover some prominent faults and appear to pond against the large fault scarp on the right edge of the picture. The unprocessed image is direct from the TOBI recorder. The image width is -6 km. (c) Postprocessed image of a large (-10 km2) lava flow in the axial valley of segment A2. Note the apparent chain of volcanic mounds just left of the plate boundary trace in the middle of the picture. The image width is -10 km.

Eos,Vol. 86, No. 22, 31 May 2005

E 2 6 0 0

Distance (km)

Fig. 2. Methane data from the "Nibelungen Field"area, along a south-north transect from 8°20' to 8°15^,S at ~13°30'W Black dots indicate water samples used to construct the Figure. Methane values are presented as nmol/L; white lines denote surfaces of constant density (isopycnals) in sigma units (kg/m³ in excess of the density of pure water).

Page 212

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Fig. 2. Schematic diagram of the climate and biosphere (CLIMBER) Earth system model of inter­

mediate complexity (EMIC). EMICs are part of a hierarchy of Earth system modeling approaches to be considered in AIMES. Adapted from Petoukhov et al. [2000J.